lets say your mce textarea instance is:

<textarea id=editor1 ....></textarea>

then you get the content as follows:

var content =  tinyMCE.getContent('editor1');

if you mean you have multiple instances of mce editor on one page and you want to get content then try this approach:

var inst, contents = new Object();

for (inst in tinyMCE.editors) {

    if (tinyMCE.editors[inst].getContent)

        contents[inst] = tinyMCE.editors[inst].getContent();

}

the above code adds each editor content into an array